1. Field of the Invention
The present invention relates to the field of semiconductor processing materials, and more specifically to a material and a method of manufacturing parts which have minimal outgassing and minimal wafer contamination during high temperature thermal steps.
2. Background of Invention
Prior art components used in high temperature semiconductor processing applications which directly contact wafers have typically been made from quartz. Quartz is clean and cheap, but it lacks the mechanical stability required to support large loads of semiconductor wafers. As wafers which are larger and heavier are being used within the industry, quartz is less desirable as a material due to its lack of strength, and particularly its lack of strength during high temperature operations. Also, quartz components have a relatively short lifetime due to warpage. Warpage of the quartz components causes the dimensions of the quartz component to deviate from the strict dimensions required for robotic handling. In addition, vertical furnaces are increasingly being used in semiconductor processing applications. The structural strength required by vertical furnace components are greater than those required for horizontal furnace components.
One solution for the lack of structural strength of quartz components is to use silicon impregnated silicon carbide (Si/SiC) composite materials. Though Si/SiC composite materials have the necessary structural strength for furnace applications, these materials typically suffer from problems associated with contamination. One form of contamination is outgassing. Outgassing is the release of vapors from the material during high temperature process steps. These vapors contain contaminating species which contaminate the wafers being processed. Furthermore, direct contamination of wafers may occur when the wafer comes into direct contact with a Si/SiC component. In the semiconductor wafer processing industry the cleanliness of wafers is a major issue. Even small amounts of impurities can create defects within the integrated circuits formed upon the wafer.
One way that manufacturers have overcome this problem is to use assemblies of components which include wafer holding "cantilevers" and "liners" which are made from Si/SiC in conjunction with quartz wafer holding containers or "boats." In these assemblies, the wafers never directly contact Si/SiC material, and the assembly has the necessary structural strength due to the structural strength of the Si/SiC parts. This approach works well with horizontal furnaces. However with vertical furnaces this approach is not feasible due to design considerations. These design considerations include, in particular, the space restrictions within the furnace and the need to design an assembly which has the necessary structural strength, is compact, durable and easy to manufacture.
Thus, it is desirable to use components made of Si/SiC which may be used in both horizontal and vertical furnaces. The design of these components may incorporate direct contact between the Si/SiC material and the wafers to be processed. This contact causes direct contamination of wafers at those portions of the wafer which touch the Si/SiC materials. This direct contact in conjunction with the outgassing associated with prior art Si/SiC material give unacceptable contamination of wafers.
Though 99.99% pure silicon carbide and electronic grade (99.9999 or better) silicon impregnate may be used to make the Si/SiC component parts, the level of metallic impurities is still to high for future and even current integrated circuit manufacturing requirements.
One cleaning method integrates a wet or dry etch process for cleaning the SiC matrix after sintering to reduce the level of impurities. This method may include the use of a mixture of acids such as HF, HNO.sub.3, and HCl. These methods seek to dissolve impurities on or near the surface. These methods are not effective since the SiC does not react with acids.
In an effort to further reduce the level of impurities outgassing into the furnace atmosphere, manufacturers have used high temperature post-process treatment or protective coatings. Both of these options have a limited lifetime. However, the use of coatings is less than a satisfactory solution as the effectiveness is highly dependent on the quality of the coating and since outgassing may occur through pin-holes and grain boundaries, outgassing from the exposed Si/SiC material is not entirely eliminated. Furthermore, the coating has a limited lifetime and peeling of the coating presents micro-contamination problems in terms of particulates. High temperature post-process treatments typically involve running the parts through a series of high temperature cycles so as to induce outgassing before the parts are used in conjunction with wafer processing. As these steps merely remove a small amount of the impurities just from the surface layers, they are not satisfactory.
Though these methods are effective in partially decreasing outgassing for at least some period of time, none of these methods yield a material which is clean enough for direct contact with wafers or a material which will not outgas over the useful life of the Si/SiC component. What is needed is a method for forming Si/SiC composite materials which do not outgas during heat process steps and which are clean enough to allow for direct contact between the manufactured parts and wafers.